you can either try and delete the bits you don't want using a magic wand/eraser/quick mask or you could use a large soft brush and erase it like in my screenshot (you would need to start with a slightly larger image so you don't erase any trees like i have)

this was just quick one so it looks a bit unbalanced but with some care it could look good. nice drawings btw

I photograph a lot of this stuff. It's actually better to photograph than scan it. Here's what I do for pencil:

Using artist's tape or Elmer's tack, mount the drawing to a piece of white foam core. Go outside about midday when it's mostly sunny. You want it to be bright, but not too intense. Set up the foam core on an easel or have someone hold it angled up, not with the sun shining directly on it.. Make sure there are no shadows cast on it. Shoot without the flash.

In Photoshop, depending on your version, either turn it black and white or desaturate it (same area of the menu, just slightly different functions). Then, hit auto levels.

That should give you a good starting point with few dark spots to work from. Hope that helps!

I too shoot at midday out on the balcony Then I do "Autocolor correction", then "Adjust brightness and contrast". This last one may erase the light pensil strokes, if I increase both the brightness and contrast.

The mistake I made with the images on the page shown was I had them exposed to the sun. I will have to retake the shots. Then it should be easier.

Once you have a good grasp on that, try bringing the highlight slider in just a little past the edge of the histogram, and it will make those whites a good bit whiter, which should help with your edges.